There are two basic methods of providing autofocus in a digital camera. The first and most common method directly uses a lens and sensor. This is, of course, the simplest and lowest cost solution.
When a user pushes a shutter button part-way down, the camera takes a series of exposures while moving the lens through the focus range. A region of the sensor is read out, and some sort of focus “figure of merit” value is generated. The camera tries to identify the peak of this signal, signifying the position of best focus.
The focus value is derived from the sharpness or “contrast” of the image in the focus region. This can be generated in the spatial frequency domain as a measure the high frequency content. It can also be calculated in the spatial domain as the sum of differences of neighboring pixels. Often the measurement is made in both horizontal and vertical directions to make best use of whatever spatial contrast exists in the focus zone.
The problem with these contrast methods of autofocus is that they rely on a single, or “unipolar” measure of focus. The camera must find the peak of this unipolar signal. Ideally, the camera would sweep though the entire focus range, but this would be unacceptably slow. An alternative is to move from far focus inward, and try to identify the peak after just passing it. This technique is susceptible to false peaks in some scenes, resulting in focus failure.
A second autofocus technique relies on technology developed for film cameras. A separate autofocus module looks at a focus zone in the scene, either though the main lens or through separate optical components. These optical components include optical elements such as prisms or beamsplitters, and special sensor arrays. A focus error signal is generated with sensor and processing circuitry within the autofocus module.
The separate autofocus modules described above generate a “bipolar” focus error signal. This gives not only a measure of focus error, but a direction. The camera moves in the direction that reduces the error signal to zero. This is faster and more robust than using unipolar focus measurements.
However, what is needed is a way to generate a bipolar focus signal using the main lens and sensor. This would have the performance of the dedicated modules without incurring the increase in cost and complexity.
Holographic elements have heretofore been used in camera auto-focusing systems. For example, U.S. Pat. No. 5,471,046 issued to Meyers discloses a camera auto-focusing system with a designator using a volume holographic element. U.S. Pat. No. 5,569,904 issued to Meyers discloses a multispot autofocus system, usable with a camera, includes a radiation emitter for emitting visible or infrared radiation. U.S. Pat. No. 6,381,072 issued to Burger discloses a stacked array magnifier that forms a magnified, demagnified or unit image of an object.
U.S. Pat. No. 5,978,607 issued to Teremy et al. discloses a photographic, digital or video camera including at least one sensor, and a viewfinder including a holographic element. U.S. Pat. No. 5,212,375 issued to Goto et al. discloses an imaging system having a focus detecting device that performs focus detection by detecting an output signal indicative of intensity distribution of light derived from a light-receiving element, in which at least one holographic optical element is arranged on the object side of a primary imaging plane to form an image on the light-receiving element. U.S. Pat. No. 4,993,789 issued to Biles et al. discloses a polarization-selective holographic element having first and second holographic layers.
However, none of the above-cited patents disclose or suggest the use of a holographic element for generating one or more displaced images that are processed to produce a bipolar contrast signal that indicates the direction of best focus and thus improve the autofocus performance of a digital camera.